1. Field of the Invention
The invention relates to a method of determining a spectrum from at least one magnetic resonance signal which is generated in a volume part of a body which contains a first and a second type of nucleus and which is arranged in a steady, uniform magnetic field, the resonance signal being obtained by polarization transfer from nuclear spins of the first type of nucleus to nuclear spind of the second type of nucleus in a first and a second pulse sequence of rf electromagnetic pulses, which pulse sequences coincide at least partly, the first pulse sequence containing a first, a second and a third rf pulse, the second pulse sequence containing a fourth and a fifth rf pulse, the pulses being separated from one another by waiting periods.
The invention also relates to a device for determining a spectrum from at least one magnetic resonance signal, which device comprises means for generating a steady, uniform magnetic field, means for generating, utilizing rf electromagnetic pulses, resonance signals in a volume part of a body by polarization transfer from nuclear spins of a first type of nucleus in the body to nuclear spins of a second type of nucleus in the body, means for generating at least one magnetic field gradient, means for receiving, detecting and sampling the magnetic resonance signal, and display means for displaying the spectrum, and also comprises processing means which include programmed arithmetic means for determining the spectrum from the sampling values obtained by means of the sampling means.
2. Description of the Prior Art
A method of this kind is known from the article "Localized .sup.13 C NMR Spectra with Enhanced Sensitivity Obtained by Volume-Selective Excitation", W.P. Aue e.a., Journal of Magnetic Resonance 61, pp. 392-395, 1985. According to such a method a body to be examined is arranged in a steady, uniform magnetic field B.sub.0. The body contains a first type of nucleus, for example a proton .sup.1 H, and a second type of nucleus, for example a carbon isotope .sup.13 C. According to said method, first a volume part of the body is selected on the basis of nuclear spins of protons for which suitable localization of proton magnetization can be achieved within the volume part because of the small chemical shift range of bound protons, after which polarization transfer is applied in order to transfer the localized proton magnetization to the .sup.13 C nuclei coupled to the protons within the same volume part. In comparison with conventional .sup.13 C spectroscopy, this offers the advantage that the sensitivity is increased and the localization of the so-called .sup.13 C sensitive volume is more accurate. For the polarization transfer after the volume selection, the body is exposed to a first and a second pulse sequence of rf electromagnetic pulses. The first pulse sequence, acting on .sup.1 H, successively comprises an rf pulse (90.degree.), a second rf pulse (180.degree.) and a third rf pulse (having a pulse angle to be varied). The second pulse sequence, acting on .sup.13 C, comprises a fourth rf pulse (90.degree.) and a fifth rf pulse (180.degree.). The pulses for .sup.1 H and .sup.13 C are applied to the body via separate transmitter coils in a device for determining a spectrum from at least one magnetic resonance signal. The fourth rf pulse coincides with the second rf pulse and the fifth rf pulse coincides with the third rf pulse. The waiting periods between the pulses amount to (2J).sup.- s in said article, where J is a coupling constant between nuclear spins of the first and the second type. After a waiting period (2J).sup.- subsequent to the fifth rf pulse, a resonance signal is obtained for bound .sup.13 C, which signal provides a spectrum after sampling and Fourier transformation. In order to achieve an adequate signal-to-noise ratio, it will be necessary to average a large number of resonance signals. The volume selection on protons is performed with VSE (Volume Selective Excitation), being a pulse gradient sequence in which three magnetic field gradients G.sub.x, G.sub.y and G.sub.z in an xyz-coordinate system are successively activated, the field direction thereof being coincident with the direction of the field B.sub.0, their gradient directions extending mutually perpendicularly, combined selective and non-selective rf pulses being applied during the application of the gradient (45.degree. selective, 90.degree. non-selective and 45.degree. selective). The first gradient G.sub.x selects a slice of the body (longitudinal magnetization exists only within the slice); the second gradient G.sub.y selects a bar within the slice and, finally, the third gradient G.sub.z selects an xyz volume part of the body. The method can be used for a CH.sub.n spin system, i.e. a body which contains molecules with CH, CH.sub.2 and CH.sub.3 groups. Because of the large number of resonance signals required, the overall measuring time required for determining a spectrum will be substantial.